This invention concerns an oil recovery method for heavy oils and tar sands wherein carbon dioxide is injected prior to and during in situ combustion operations.
It is well recognized that primary hydrocarbon recovery techniques may recover only a portion of the petroleum in the formation. Thus, numerous secondary and tertiary recovery techniques have been suggested and employed to increase the recovery of hydrocarbons from the formations holding them in place. Thermal recovery techniques have proven to be effective in increasing the amount of oil recovered from the formation. Waterflooding and steamflooding have proven to be the most successful oil recovery techniques yet employed in commercial practice. Some successes have also been achieved with in situ combustion processes.
An in situ combustion process requires the injection of sufficient oxygen-containing gas to support and sustain combustion of the hydrocarbons in the reservoir. When the flow of the oxygen-containing gas in the reservoir is large enough, combustion will occur, either spontaneously or from another source such as a downhole heater. A portion of the oil is burned as fuel at the front which proceeds slowly through the reservoir, breaking down the oil into various components, vaporizing and pushing the lighter oil components ahead of the burning regions through the reservoir to the production wells. Some heavy oil formations can create problems for in situ combustion drives with a low permeability which makes it difficult to inject an oxygen-containing gas. A second problem which may also exist is the damping or the extinction of the combustion front caused by viscous oil banks.
Several methods have been suggested by the prior art to improve in situ combustion drives. U.S. Pat. No. 3,375,870 suggests injecting steam into a formation until breakthrough at the production wells, continuing to inject steam of a reduced steam quality and concluding with in situ combustion. U.S. Pat. No. 3,680,634 discloses the injection of water, hot water or steam prior to in situ combustion. U.S. Pat. Nos. 3,563,312 and 3,794,113 both disclose the injection of steam into a formation prior to in situ combustion. An additional reference, U.S. Pat. No. 4,099,568 suggests the injection of a non-condensable, non-oxidizing gas ahead of or in combination with steamflooding to reduce the tendency of viscous oil plugging during steam injection. U.S. Pat. No. 4,099,568, however, does not disclose the use of in situ combustion.